The present invention generally relates to fabrication methods and resulting structures for semiconductor devices. More specifically, the present invention relates to a tone inversion integration process for forming phase change memory.
Phase Change Memory (PCM) is an emerging technology for storage class memory and is also an attractive candidate for memory-based machine-learning accelerators for cognitive computing. Phase change based memory materials such as the chalcogenides (alloys of elements of group VI of the periodic table) can be caused to change phase between an amorphous state and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher electrical resistivity than the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using programmable phase change based materials to form nonvolatile memory circuits, which can be read and written with random access.
PCM materials typically include a composition of elements (e.g., two to six), and the functionality of a PCM material is highly dependent on the particular composition of elements that is selected. In a storage element that relies on PCM, data is stored by causing transitions in an active region of the phase change material between amorphous and crystalline states. For a one bit cell, the difference between the highest resistance of the low resistance crystalline set state and the lowest resistance of the high resistance amorphous reset state defines a read margin used to distinguish cells in the crystalline set state from those in the amorphous reset state. The data stored in a memory cell can be determined by determining whether the memory cell has a resistance corresponding to the low resistance state or to the high resistance state, for example by measuring whether the resistance of the memory cell is above or below a threshold resistance value within the read margin.